fix typos: namespace and linkerd-cni across versions

## Description

Fix typos found in Linkerd documentation across multiple versions:

1. **`namesppace` → `namespace`** - Typo in iptables reference commands
2. **`linkerd-cn` → `linkerd-cni`** - Incorrect CNI namespace name in troubleshooting guides


## Impact

These typos affect documentation clarity and may cause confusion when users attempt to run the documented commands for troubleshooting.